Kevin Federline Throws Birthday Party for Sons

While Britney Spears struggled at the Video Music Awards in Las Vegas, her ex-husband Kevin Federline threw a backyard pool party Sunday – for their two sons' birthdays, PEOPLE has learned.

Personal assistants were seen picking up banners, balloons and an ice cream cake, all in the theme of the animated movie Cars, for Jayden James, who turns 1 on Wednesday, and Sean Preston, who turns two on Friday.

Spears's parents, Jamie and Lynne, stopped by for the party at Federline's San Fernando Valley home, joining about 20 guests. They had also visited their grandchildren the day before, while their daughter was hitting the club scene in Vegas before her widely panned VMAs performance.

Spears, 25, and Federline, 29, are embroiled in an increasingly fierce custody battle over their boys. They currently have 50-50 custody, but Federline is seeking more time, serving subpoenas on people in Spears's inner-circle to show problems in her parenting skills.